Thursday, 28 May 2015

I have just finished sheathing the stepper motor cables and wiring the end connector. I used 6mm diameter expanding braid sleeving to cover the stepper leads. I secured each end of this with adhesive impregnated heatshrink tubing. I'm not sure how long this will last, only time will tell.The connector is a five way locking type, obviously the fifth pin is not used. The locking feature means that the stepper cannot be inadvertently disconnected whilst being driven. The pin assignments in the connector are as follows:-

1 = White

2 = Green

3 = Black

4 = Red

I realise that the above means nothing to any of you, but the more places I write it down, the less likely I am to forget it.

As mentioned earlier, the major mechanical part of this tool build is now complete. I will still have to make a hand wheel for the other end of the stepper shaft. This will allow manual operation of the table if required.

Here is a photograph of the (almost) complete motorised rotary table:-

And here is a view of the end connector:-

Tomorrow I will begin boxing up the electronics. Now, the bare minimum of front panel buttons would be five. These would be UP/DOWN LEFT/RIGHT and SELECT. Various trial button layouts on the front panel always looked cluttered, so I wanted to find a simpler solution.

You already know that I like a recycled component or two. Rummaging through my stock of useful parts, I found some small spring return to centre SPDT switches. So I can use one each of these for UP/DOWN LEFT/RIGHT and then one push button for SELECT.

If only...The four holes to mount the stepper have now been drilled in the square end plate, and the stepper and mounting have been successfully mated up to the rotary table.I decided to use M5 nuts and bolts to attach the stepper, as I've never been too comfortable with the idea of threads tapped into aluminium. So the holes ended up being 5.1mm diameter.Here is a photograph of the stepper motor and mounting connected to the table:-

I ran up the stepper and shifted the rotary table round under power; the system worked perfectly.

Here is a photograph looking along the inside of the mounting, showing two of the table attachment screws, as well as the coupling:-

That is the mechanical portion of this tool build completed, now it is over to the electronics - ruggedising the motor leads and fitting all the PCB's into a die cast box.

Wednesday, 27 May 2015

I have just finished milling the coupling access slot. I milled it slightly off centre. This makes getting to the offset grubscrews in the coupling somewhat easier.Here is the part showing the slot and slipped over the rotary table drive sleeve, but not bolted on. You can see the bellows coupling peeking through it's newly created "window":-

Hopefully sometime today I will get the stepper mounting holes drilled and tapped to M5. That will then be the part finished. It is a good job as I am rapidly running out of Step related punning post titles...

...pation.The experiment in bonding seems to have worked. After almost 18 hours of cure time - not strictly required but entirely due to my paranoia - the mounting is solid. Will it last with the weight of the stepper motor on it, and with general handling? That remains to be seen. All seems well so far, at least.Here is a photograph of the mounting to show the three 120 degree spaced holes. These are 5.1mm, to suit the M5 attachment screws:-

And here is the part fitted to the rotary table:-

The next task will be to drill the mounting holes for the stepper motor in the square portion, then mill the coupling access slot. After that it will be time to assemble the electronics in the die cast case. Keep watching.

Tuesday, 26 May 2015

I'm sitting writing this post having just returned from the workshop. This evening I drilled the three 120 degree spaced bolt holes that enable the motor mount to be fixed to the rotary table. I know what you are thinking...how do you drill three holes on a pitch circle without a rotary table (because your rotary table is in pieces...)? Well, it was a simple matter of removing the threaded portion on the table and using it as a drilling template. The work of a few minutes.I said yesterday that I was moving away from using welding to join the components. An aluminium TIG weld bead is a broad thing at the best of times, and I feel that it would detract from the look of the part, as well as possibly being overkill.This is quite a departure for me, since I would normally look to my TIG welder before any other option. I have decided, in this instance, to use two part epoxy. I mixed some up and wet assembled the three parts with it. Here is a photograph of the unit clamped in the milling vice while the epoxy cures:-

I have to admit a certain level of anxiety about this experiment in bonding; will the epoxy have sufficient mechanical strength to do the job? I see you shiver with antici...

Monday, 25 May 2015

I got the round portion of the motor mounting bracket finished today. This started life as a piece of 2 1/4 inch 6082 bar. I turned this down to 56mm in the four jaw, then bored a 21mm hole in the centre to suit the rotary table driveshaft outer sleeve. Once the central hole was complete, a 3mm deep recess was bored to suit the cylindrical portion described in the last post.Here is a photograph of the round section showing the central hole and the locating recess:-

Here are the parts made so far:-

And here they are assembled:-

The next tasks will be to drill the mounting holes in the round section and then mill the coupling access slot in the tubular. When it comes to the final assembly, I am starting to edge towards joining the components together with Loctite rather than welding them. All that will be a job for tomorrow!I did some more work on the design of the tubular chamber concept yesterday. I will post this in the next few days.

Saturday, 23 May 2015

I have just finished another portion of the motor mounting bracket destined for the indexer. A section of 2 inch x 10swg 6082 aluminium tube was set up in the four jaw and turned down to exactly suit the register in the square plate. Having turned the tube sides, I then faced the end to ensure it would be square with the sides.

The tube was then parted off. The length was decided by the amount of room required to mill an access slot to enable me to get at the bellows coupling grub screws, once everything is assembled. It ended up being near enough exactly 67mm.

Here is a photograph of the square plate and tube trial assembled:-

The next two pictures illustrate the fit up of the motor and the mounting hardware so far produced:-

I should get all the machining finished by tomorow, with any luck. Will I run out of step related titles for my posts? Stay tuned to find out.

Friday, 22 May 2015

This evening I started machining the parts for the stepper motor mounting bracket, to complete the indexing system.The stepper will be mounted to a square plate fixed to a tube which transitions to a round plate screwed to the existing rotary table flange. My first action was to cut a piece of 6mm thick aluminium plate with a jigsaw. I keep promising myself a plasma cutter...it has to happen at some point! Next I squared this up to size on the milling machine. The plate is 56mm x 56mm, this being the standard NEMA 23 end face dimension.Next I bored a 38mm hole through the plate in the four jaw. The idea is that this will act as a locating feature for the 38mm register on the front face of the motor. Still located in the four jaw, the plate was then counterbored to 48mm to a depth of 3mm. This was done to locate the tube that will join the square and round sections of the mounting. I will be welding the whole assembly together once the other parts are made.Here is a photograph of the part thus far:-

I will hopefully get a chance to progress the mounting a bit further tomorrow. Keep checking for updates. Building this tool has taken me away from the actual design of the tube bundle chamber, so I intend devoting some time to that over the coming days.

Today I got the stepper motor connected to the rotary table input shaft for a preliminary test. The mechanical link between the stepper shaft and table shaft was made with a bellows type aluminium coupling. The stepper drive shaft is 12mm diameter, whilst that of the motor is 6.35mm. i.e. 1/4 inch. I managed to find an off the shelf bellows coupling with these bore dimensions.Due to the relatively high gear ratio of 90:1, I took the decision to dispense with micro-stepping of the motor, and I altered the appropriate line in the software to reflect this, as well as setting the dip switches on the driver circuit.Here is a picture of the set up. As can be seen, the motor was supported by a machine table clamp during the test:-

The motor ran the table with no trouble at all. I ran the table in Step, Angle, Jog and Run modes. I am not sure if the motor will be able to drive the table for actual machining, as opposed to just positioning, but it certainly did not seem to lack torque.I didn't attempt any accurate measurement of the angle the table turned through in this test. That said, going by the graduations on the side of the table, everything seemed to be in order.The metal has now arrived to fabricate the mounting bracket, so look out for another update in the next few days.

Monday, 11 May 2015

I suspect that it will be abundantly clear to you, my astute reader, that the production of a tube bundle rocket thrust chamber will require the fabrication of a foundation element with a multitude of holes equally spaced on a PCD.Of course this means indexing. The usual way to achieve this is to use a rotary table with dividing plates. This time honoured method, whilst working extremely well, is also rather tedious and is prone to human error - especially when I am doing it!Therefore I decided that it would be worthwhile spending a little time motorising my Vertex 6 inch rotary table to automate the indexing process.After some internet research I discovered Gary Liming's excellent Step Index project. Gary writes for the US magazine "Digital Machinist". Gary also has a very good website with a build log for the project.Step Index is an Arduino based system with the firmware currently at version 2.3. The system can work with fixtures with a variety of gear ratios. Initially it was designed for a 3:1 set up, but has since been re-written to cope with 40:1 and 90:1. Most rotary tables are 90:1, as is mine. The Arduino source code can be downloaded from the Digital Machinist site, as can a Read Me file explaining how to make modifications to the code to add more ratios or to make a specific value the default.Gary gives a comprehensive explanation of the operation of his system on his site, so I will only briefly outline it here.An Arduino Uno board is at the heart of the system. This is fitted with an LCD keypad shield. I got mine from Sain Smart, but there are many other sources for this part. The Arduino displays system modes and status via the LCD. A stepper driver board based on the Toshiba TB6960 IC is clocked by the Arduino, thereby driving the stepper motor. The motor I used is from Zapp Automation here in the UK, type SY57STH76. This is a NEMA 23 frame, 2A, 1.8 degree per step motor with a holding torque of 1.89Nm. I also obtained a 12V, 5A switch mode supply to run the lot.The photograph below shows the system connected up for bench testing:-

The buttons on the LCD shield are used to scroll through and select menu items. I will piggyback larger, more user friendly ones when I fit the system into a die cast case. The menus are as follows:-Ratio - Allows the user to select the ratio of the fixture being used. The software can be modified to include as many as are required, but 3:1, 40:1 and 90:1 are included as standard.Temp - Facility to read motor and driver heatsink temperature, using two sensors connected to the Arduino's analogue inputs. I did not use this function.Step - Enables user to specify a number of divisions. The motor is then incremented the correct number of steps for each division.Angle - Similar to above except that the user inputs the angle the fixture is to be rotated through.Run - The motor will run continuously with speed set by the user.Jog - The motor can be nudged a preset number of steps.The photographs below show the Arduino Uno board and LCD shield, the stepper drive board and the switch mode PSU.

The momentary push buttons can be seen here, as can the multi-turn potentiometer used to set the screen contrast.

Stepper driver board. DIP switches allow the setting of current characteristics and microstepping settings. The power IC and heatsink are on the underside.

The switch mode PSU. The preset potentiometer above the terminal strip allows the output voltage to be set exactly.

The next step is to prepare the rotary table. This unit is a Vertex HV6, and I've posted pictures of it on this blog before. It will need to be finessed somewhat before the motor can be fitted. I will also have to make the motor mounting hardware.

Once assembled the new improved rotary table will greatly facilitate the machining of the engine components.

Friday, 8 May 2015

More on telemetry...It seems fairly obvious that a device as potentially fractious as a home built liquid fuelled rocket engine would be best viewed from a safe distance. Indeed, Krzycki suggests viewing the rocket exhaust plume by means of a conveniently positioned mirror.Fortunately things have moved on since 1967. From the outset, a video system was always going to be a part of the telemetry set up I had in mind. Nowadays, small, reliable CCD cameras and USB digital video recording systems (DVR's) can be had for a modest outlay.I purchased a four channel USB DVR and four miniature CCD cameras from Lightinthebox.com. The cameras are also equipped with a small microphone. Each camera has a flying lead terminated in three RCA type connectors. The colour coding used is as follows:-

Red = 9VDC power

Yellow = Video signal

White = Audio

The DVR features four video inputs through male BNC connectors, hence a BNC to RCA adaptor was required for each one. There is also one audio input. This is not a great worry as I do not anticipate hearing the engine being a problem.

Each camera measures ~ 20mm (0.75") square. They are thus small enough to fit into a tight space.

Here is a photograph of the DVR unit with all four cameras connected:-

In and amongst this snake's wedding it is possible to see the four cameras, DVR and the RCA to BNC adaptors.

The DVR comes packaged with software to view and record the camera images. The images can be viewed in quad format, or each channel can be viewed separately. Motion detect record is also possible. Here is a screenshot of the software running, with some frankly resistable images of my kitchen:-

And here is a close up of one of the cameras, the lens through the lens, so to speak:-

When these cameras are spread around the test stand, they will greatly improve the safety of operation, as well as acting as a back up and cross reference to any data acquisition set up. Transducers may well tell the story, but a recording of a camera pointing at a bank of gauges is still hard to beat.

And finally...on the subject of telemetry, National Instruments have just released a home version of their industry leading LabVIEW software, aimed at the Maker Movement. It boasts elements compatible with Arduino, and is being offered at a bargain price. Google "NI LabVIEW Home bundle" and you will see what I mean.

About this blog

This blog describes the research, design and construction of a Liquid Rocket Engine.
As such it will include information regarding the design and construction of rocket engine components.
This will encompass theoretical and performance concerns, as well as machining, welding and manufacturing techniques used to overcome the various problems encountered.
In addition, my interests in this direction include control and data acquisition. So there will be posts regarding electronic systems and microcontrollers.
It is my hope that as well as being of interest to the rocket engine community, it should also become a repository of general amateur engineering information.
I was inspired to create a blog by the groundswell of interest that I have had in my project from people I have met. I have found that their reactions tend to go from perplexity to enthusiasm rapidly! The main question most people have is not to do with the technical obstacle to be overcome. Most of those who have asked me about my project have wanted to know "Why are you doing this?" So I will try to give some answers to this and to explore my motivation to think, research, create and construct.